There have been relatively few randomized clinical trials of pharmacology for attention-deficit/hyperactivity (ADHD) in children with Autism Spectrum Disorders (ASD). Studies involving stimulant medications have generally found poorer response rates and higher rates of intolerable side effects than in typically developing children with ADHD. Atomoxetine (Strattera) was approved by the FDA in January 2003 for treatment of ADHD. A nonstimulant, it inhibits the presynaptic norephinephrine transporter. There have been only four studies (three open and one small double-blind) examining this medication in children with ASD. Behavioral treatments, such as training parents in the use of behavioral interventions (PMT), are also standard treatments for ADHD in typically developing children. While PMT also has long been used in ASD, it has rarely been studied in combination with psychopharmacologic treatment in this population. This proposal addresses four objectives: a) to conduct a double-blind, placebo-controlled, 10-week parallel-group comparison of atomoxetine (ATX) versus placebo on i) clinician- and parent-rated behavior, ii) direct observations of behavior, iii) cognitive/performance measures of attention, and iv) adverse events in 156 children with ASD (78 treated with ATX, 78 with placebo); b) to conduct a 10-week parallel-group comparison of PMT versus no-PMT on behavioral measures of compliance in 156 children with ASD (78 with PMT, 78 without PMT) c) to conduct a 10-week parallel-group comparison of combined (ATX plus PMT) vs. single treatment (PMT alone) on measures of compliance in 156 children with ASD; and d) to compare the long-term maintenance of efficacy, safety, and tolerability of ATX and PMT via an open-label 6-month extension trial of responders. Medication doses will be titrated against benefit and side effects over the first 6 weeks (ATX to a cap of 1.8 mg/kg/day). PMT will be 10 sessions, 1-1.5 hr. each, modeled on the RUPP Autism Network PMT. Data analyses will capitalize on the power of a 2 x 2 analysis where appropriate. Children with ASD have been neglected in pharmacological research, placing an enormous burden on health care and educational systems. This trial has the potential to provide answers regarding optimal treatment (both psychosocial and pharmacological) for a sizeable subgroup of children with ASD who experience symptoms of over activity and inattention.